From a tribological standpoint, a combination of high mechanical hardness with low friction is always desired but seldom achieved in most sliding surfaces. With many conventional coatings, base steel sliding against itself gives friction coefficients of 0.12 to 0.16 even under the best lubrication conditions.
Alloying ingredients used in certain coatings are inherently soft and therefore possess fairly good solid lubrication properties. Metals such as copper, tin, antimony, cadmium and zinc show good lubrication properties under oil lubricated sliding conditions as well. Specifically, these metals primarily undergo easy shear under loaded sliding contacts due to their softness. In one set of sliding experiments with pure metals under lubricated conditions, pure copper, bismuth, antimony, tin, and zinc were considered. All of the metals except bismuth resulted in a low friction coefficient, confirming that the respective metals were lubricious under lubricated sliding conditions. Additionally, some of the aforementioned metals, such as zinc, possess a good ability to react with the additives in lubricants to form chemical boundary films on sliding surfaces. These chemical films may also possess some lubrication or anti-friction properties. However, soft metals and their films have typically not been used as structural materials for lubricated applications under severe loading conditions, primarily due to their poor mechanical properties and rapid wear.
For all of these reasons, it would be desirable to develop a new and improved coating that possess an improved degree of hardness relative to conventional coatings while also possessing improved lubrication properties.